The present invention relates to a biaxially oriented polyester film. More particularly, the present invention relates to a biaxially oriented polyester film which contains crosslinked polymer particles having a particular particle size distribution ratio, a particular average particle diameter and a particular shape and which has a flat film surface having a small number of large protuberances, excellent running property and excellent winding property.
Biaxially oriented polyester films are in wide use in various applications such as magnetic tape, electric appliances, photographs, metallization, packaging and the like, owing to their excellent properties. Owing particularly to their high strength and elastic modulus, they are in wide use as a base film for a video tape, an audio tape, a computer tape, a floppy disk, etc.
With increasing demands for high-density recording and high quality in these application fields in recent years, a polyester film used as a base film is increasingly required to have flat surface(s) while retaining running property.
In order to improve the running property of a film, inert inorganic particles are added to a polyester to form fine protuberances derived from the particles on the film surface at the time of film formation. Inert inorganic particles usually have a wide particle size distribution and contain coarse particles and must, therefore, be subjected to classification to remove the coarse particles, before the addition to the polyester. This classification, however, incurs increase in production costs, and the cost drastically increases as the classification is made to a higher degree). Moreover, the classification has a limit in narrowing the particle size distribution and it is unable to remove the coarse particles completely.
Hence, researches have been made in recent years on industrial production of particles, particularly crosslinked polymer particles, containing no coarse particles and having uniform particle diameters, and some of such particles are in industrial production. These particles, however, have a relatively large average particle diameter. Production of particles having uniform and smaller particle diameters involves problems to be solved and requires a higher production cost.
The research by the present inventors revealed that in producing a biaxially oriented polyester film of flat surface using the above-mentioned inorganic particles or crosslinked polymer particles, the running property of a film can be increased to a certain degree but the film can not be wound into a film roll in a good form in some cases and that this problem (a film roll in an inferior form) is more serious in using crosslinked polymer particles of uniform particle diameters and is invited by the shifting of film toward left or right in high-speed winding of the film to cause uneven sides of film roll. The reason for the problem is presumed to be that the protuberances on the film surface have relatively uniform heights and this allows air to be trapped between film layers when the film is wound into a film roll. In order to obtain a film roll of good form having even roll sides, it is therefore necessary to minimize the amount of air intake and further to permit easy removal of the trapped air from between film layers.
In order to achieve the easy removal of trapped air from between film layers, it is effective that the protuberances on film surface are allowed to have nonuniform heights by using particles having a relatively wide distribution of particle size. In this respect, the above-mentioned inert inorganic particles are superior to crosslinked polymer particles having uniform particle diameters. The inert inorganic particles, however, contain coarse particles, allowing the protuberances on film surface produced by such coarse particles to have too large heights, and as a result, the flatness of film surface is lost. This brings about reduction in electromagnetic conversion characteristics when the formed film is made into a magnetic tape, or the protuberances produced by the coarse particles cause drop-out. Thus, the increasing demands for high quality polyester film can not be satisfied by use of inert inorganic particles.
The present inventors made an extensive research in order to develop a biaxially oriented polyester film which is free from the drawbacks of conventional inert inorganic particles or crosslinked polymer particles and which has a flat surface having a small number of large protuberances and, in high-speed winding, generates neither wrinkle nor unevenness of film roll sides. As a result, it has been found that such a film can be obtained by allowing a biaxially oriented polyester film to contain an appropriate amount of particles of particular shape obtained by grinding crosslinked polymer particles having a particular particle size distribution ratio and a particular average particle diameter. This finding has led to the present invention.